1. Field of the Invention
The present invention relates to the field of communications. More particularly, the present invention relates to cellular radiotelephone communications between an airplane and a ground based station.
2. Description of the Related Art
Present communications technology enables aircraft passengers to make telephone calls to anywhere in the world from any properly equipped airplane. Large airline-type aircraft as well as smaller general aviation-type aircraft can be equipped with the radiotelephone equipment.
A radiotelephone conversation is typically accomplished by first entering the telephone number to be called as well as credit card information to pay for the call. The radiotelephone then connects with one of 70-80 radiotelephone base stations, also known as cells, on the ground. The cell to which it connects depends on to which base station the aircraft is closest when the call is initiated. The cells, each connected to the public switched telephone network (PSTN), cover most of the continental United States, thus allowing a telephone call to be initiated from an aircraft almost anywhere.
Aircraft radiotelephones, however, experience a number of problems. First, the aircraft based radiotelephone does not register in the ground based system. The ground based system, therefore, does not know the location of the aircraft radiotelephone. This restricts the aircraft radiotelephone to initiating calls; it cannot receive calls since the ground system does not know where to forward calls.
Another problem is that the aircraft radiotelephone system does not perform hand-offs between cells as is done in ground based cellular radiotelephone systems when the radiotelephone reaches the edge of the cell. This results in the call from the aircraft radiotelephone being dropped when the aircraft reaches the limit of the cell's coverage. There is a resulting need for an airborne radiotelephone system that is compatible with the ground based cellular radiotelephone system. In other words, an airborne radiotelephone system is needed that enables ground initiated telephone calls to be received by the airborne radiotelephone in addition to the call from the airborne radiotelephone being handed off to the next cell as it reaches the edge of the cell's coverage.